1. Field of the Invention
The present invention constitutes an improved technique for performing percutaneous transluminal angioplasty. Angioplasty is a medical procedure used to treat patients whose arteries have become occluded due to the disease process call atherosclerosis.
Arteriosclerosis is a general term which refers to any of a group of diseases in which the lumen of an artery becomes narrowed or blocked. The most common and important form of arteriosclerosis, especially in Western societies, is the disease known as atherosclerosis. In atherosclerosis, there is an accumulation of lipids in the intimal, or inner, layer of the affected artery. The resulting intimal thickening restricts the flow of blood so as to hinder the functioning of, or permanently damage, the organ which the artery feeds. These accumulations of lipids tend to be localized and can occur in coronary, cerebral, or peripheral arteries. They will hereinafter be referred to synonymously as lesions, plaques, or atheromas.
The lipid accumulation is made up of free lipid and smooth muscle cells which have proliferated and taken up lipid. As the disease progresses, the lesion may begin to absorb calcium which causes it to harden and may also be composed of blood which has clotted in response to the presence of the atheroma. Although the process of plaque formation is not completely understood, it is known to be progressive, and atherosclerotic plaques may vary greatly in their physical characteristics.
Treatment of atherosclerosis is aimed at alleviating the diminished blood flow. This can sometimes be done by medical means which cause the smooth muscles of the arterial walls to relax and thereby dilate the artery. Other treatment methods are directed toward physiological compensation for the reduced blood flow. In cases where the artery is severely occluded, however, there is no reasonable alternative but to try to re-establish a lumen of proper diameter. A number of surgical procedures have been developed toward this end. These include endarterectomy, in which the plaque is surgically removed, and by-pass grafts, in which a segment of artery or vein from elsewhere in the body is removed and reattached in place of the occluded artery. These procedures are major surgical operations and present a number of disadvantages to a patient including financial cost, inconvenience, and the risk of complications associated with any major surgery. Therefore, in the past several years, methods of re-establishing the patency of an occluded artery have been developed which are relatively non-invasive and present less risk to a patient than conventional surgery. One such method is transluminal angioplasty.
2. Description of the Prior Art
The conventional method of performing transluminal angioplasty uses a special double lumen catheter. The first, or inner, lumen allows passage of a guide wire. Concentric with this lumen is a second lumen which connects to a sausage-shaped segment or balloon at the distal end of the catheter. The second lumen and balloon are generally filled with diluted contrast media. Contrast media is radio-opaque liquid which makes visualization of the catheter possible by means of X-rays. The procedure first involves selecting a convenient place to introduce the catheter into the arterial system of the patient, such as the femoral artery of the leg. Next, the catheter is guided to the blocked artery. This is done manually and with the aid of an X-ray monitor. When the catheter is appropriately positioned, the guide wire is advanced to and past the point of obstruction. The balloon catheter, which surrounds the guide wire, is then advanced along with the guide wire until it is surrounded by the occluding plaque. The balloon, made of material with high tensile strength and low elasticity, is inflated to a pressure as high as twelve atmospheres. As the balloon expands it creates a larger inner diameter within the occluded artery. It is not known with certainty what physical processes occur within the occluded artery in response to the balloon inflation, but the usual method is to inflate the balloon to a certain predetermined pressure and repeat the inflation an arbitrary number of times. The balloon is then collapsed and retracted. The site of the obstruction is then examined angiographically and, if the artery is still occluded, a decision is made either to repeat the angioplasty procedure or to resort to some other option.
As aforementioned, the procedure involves inflating the balloon to a predetermined pressure. Although the operator may observe the size of the balloon during the inflation by means of the X-ray monitor, unless the pressure is measured, the bursting pressure of the balloon may be exceeded causing rupture. Therefore, practitioners have realized the need for continuous monitoring of the fluid pressure within the balloon. As it is conventional to inject fluid into the balloon with a syringe, the most obvious method is to interpose a T-fitting between the delivery end of the syringe and the balloon catheter. A standard pressure transducer can then be connected to the T-fitting and the fluid pressure within measured. U.S. Pat. No. 4,370,982 discloses a method for measuring fluid pressure without the transmitter coming in contact with the working medium. The '982 patent also discloses an injection device which uses a threaded member which when rotated produces translational motion of the syringe plunger. The relatively slow inflation is supposed to reduce further the risk of balloon rupture.
Another relevant patent is U.S. Pat. No. 4,446,867 which discloses a method and apparatus for generating pulses of pressure within the balloon catheter. As discussed above, some atheromas become hard due to calcification and therefore resist dilation by the balloon. The '867 patent represents an attempt to deal with this problem by inflating the balloon so rapidly that the plaque is broken. Although the specification of the '867 patent recites that pieces of broken plaque will be removed by normal cardiovascular processes, it seems obvious that such fragments may flow downstream and become lodged in a smaller artery, thereby completely blocking blood flow. As pieces of plaque may break off during conventional angioplasty procedures, even without using the pulsed pressure method of the '867 patent, it is important to know when this has occurred so that remedial steps may be taken.